Esr and Ir Evidence for Chromium in Kaolinites

Evidence obtained from ESR and IR studies is presented for the presence of structural Cr in two natural kaolinites (MILO and GEY) formed in an hydrothermal environment in Sonoma County, California. The XRD patterns show a greater structural disorder for GEY than for MILO, but both have the usual hexagonal shapes as shown by TEM observations. On the basis of EDX analysis of different particles, GEY, on average, appears to be richer in Cr (2.1% Cr2O3) than MILO (0.6% Cr2O3) The presence of Fe oxide particles containing some Cr, Ni and V is also indicated by EDX analysis. By FT-IR observation, the octahedral Cr3+ position was easily detected by a well resolved spectral feature at 3586 cm(-1). The adsorbed Cr3+ kaolinite (KMRXCR) presents no band at 3586 cm(-1), but two other bands situated at 3527 and 3477 cm(-1). The main features of the ESR spectra of these two kaolinites are a set of resonances near g(eff)4; a broad resonance centred near g(eff)2 with some modulations; and a set of resonances near g(eff)1. The broad resonance centred near g(eff)2 is interpreted as the free iron oxide signal with modulations due to VO2+. The set of resonances near g(eff)4 is similar to that observed for octahedral Fe3+, but the position is shifted compared to that of octahedral Fe3+. This set of resonances near g(eff)4 is, therefore, interpreted as belonging to Cr3+ in octahedral position. The resonances at g(eff)1 are also interpreted as belonging to Cr3+. Comparison with the Cr3+ surface-adsorbed ESR signal of a Cr-saturated kaolinite (which is a broad resonance centred near g(eff)2) strengthens the interpretation of the g(eff)4 resonances belonging to Cr3+ in the octahedral position. The results obtained by the combination of FT-IR and ESR spectroscopies indicate that Cr3+ is present in the octahedral position of the MILO and GEY kaolinites.